Dieter G. Ast

Nucleation mechanisms and the elimination of misfit dislocations at mismatched interfaces by reduction in growth area

To investigate the effect of growth area on interface dislocation density in strained‐layer epitaxy, we have fabricated 2‐μm‐high mesas of varying lateral dimensions and geometry in (001) GaAs substrates with dislocation densities of 1.5×105, 104, and 102 cm−2. 3500‐, 7000‐, and 8250‐A‐thick In0.05Ga0.95As layers, corresponding to 5, 10, and 11 times the experimental critical layer thickness as measured for large‐area samples, were then deposited by molecular‐beam epitaxy. For the 3500‐A layers, the linear interface dislocation density, defined as the inverse of the average dislocation spacing, was reduced from greater than 5000 to less than 800 cm−1 for mesas as large as 100 μm. A pronounced difference in the linear interface dislocation densities along the two interface 〈110〉 directions indicates that α dislocations nucleate about twice as much as β dislocations. For samples grown on the highest dislocation density substrates, the linear interface‐dislocation density was found to vary linearly with mesa...

Structure and recombination in InGaAs/GaAs heterostructures

The defect structure of lattice‐mismatched 1‐μm InxGa1−xAs (x≊0.12, misfit Δa/a≊8.5×10−3) epilayers on GaAs was studied with scanning cathodoluminescence (CL), transmission electron microscopy (TEM), high‐voltage electron microscopy, and scanning electron microscopy. CL shows that nonradiative recombination lines exist in the GaAs buffer layer as far as 4000 A from the interface. The density of these defects is independent of substrate dislocation density. Plan‐view TEM analysis indicates that the majority of these dislocations in the buffer layer are sessile edge half‐loops. Cross‐sectional TEM shows that loops also extend into the InGaAs epilayer, but the majority of the loops are located on the buffer layer (substrate) side of the interface. A model is proposed to explain sessile edge dislocation formation in the buffer layer. A comparison of CL and high‐voltage electron microscopy images from the same interface area reveals that the dark nonradiative recombination lines seen in scanning luminescence i...

Elimination of interface defects in mismatched epilayers by a reduction in growth area

We have eliminated interface defects from the mismatched In0.05Ga0.95As/ (001)GaAs interface by controlling the size of the growth area. 2‐μm‐high pillars with different lateral shapes and dimensions were defined within the GaAs substrate before the molecular beam epitaxial growth of 3500 A of In0.05Ga0.95As, greater than four times the critical thickness. On the pillars, the linear density of misfit dislocations was reduced from >5000 dislocations/cm for large (several hundred μm lateral dimensions) growth areas to nearly zero for 25 μm lateral dimensions. The dislocation density remains less than 800 dislocations/cm for lateral dimensions up to 100 μm. We find that there is also a decrease in dislocation density in narrow channels between the pillars; therefore, the pillars also block the glide of misfit dislocations.

Viscosity and crystallization kinetics of As2Se3

Abstract The crystallization kinetics, viscosity, and temperature coefficient of expansion of amorphous As2Se3 were investigated. The crystallization kinetics of As2Se3 are characterized under isothermal conditions for undercoolings ranging from 37°C to 122°C using thermal analysis techniques. The crystallization kinetics can be accurately described by the Johnson-Mehl-Avrami transformation equation. At least at the lower observed temperatures As2Se3 exhibits an increasing nucleation rate with respect to time. Microscopic examination of partially crystallized specimens reveals constant isothermal growth rates with respect to time and a spherulitic growth habitat for the crystalline phase. All spherulites nucleated at free surfaces. The individual As2Se3 crystallites in the spherulites have a plate-like growth morphology at temperatures well below the crystalline melting point. At temperatures approaching the crystalline melting point the crystallites assume a rod-like growth morphology. The activation energy for the crystal growth rate was determined to be 1.6 eV by direct observation. A model for the crystallization kinetics consistent with the data is suggested which yields an activation energy of 2.7 eV for secondary nucleation within the spherulites for 240°C The viscosity of As2Se3 in the pure and doped states (1 at.% Ag) was measured in the glass transition region. Utilizing these data for pure As2Se3 and the high temperature data of previous investigators the interpolated activation energy for viscous flow in the temperature region of rapid crystallization was determined to be 2.7 eV. The linear temperature coefficient of expansion was determined to be 1.55 × 10−5/°C for T

The structure of second and third order twin boundaries in silicon

Symmetric second and third order twin boundaries in silicon have been observed using high resolution TEM. Micrographs of a symmetric (221)..sigma.. = 9 boundary exhibited contrast feature consistent with the zig-zag dislocation structure proposed by Hornstra and similar to those reported in germanium. A model for a symmetric (529)..sigma.. = 27 boundary was constructed and found to have the same periodicity and faceted structure as an experimentally observed ..sigma.. = 27 boundary. A system for modelling the structure of tilt boundaries in tetrahedrally coordinated materials was developed based on the early work of Hornstra. Boundaries with (110) median planes and misorientations up to 70.53/sup 0/ were constructed using a repeating series of simple structural units.

Effects of the presence/absence of HCl during gate oxidation on the electrical and structural properties of polycrystalline silicon thin‐film transistors

The influence of the presence or absence of HCl during gate oxidation at 1100 and 1150 °C on the electrical and structural properties of polycrystalline silicon thin‐film transistors was investigated. Devices processed without HCl exhibited a lower leakage current, a larger current switching ratio, a 25%–55% increase in carrier mobility, and a 21%–30% lower grain‐boundary trap density. Materials investigation showed that the improvement was not primarily due to an increase in grain size, which was about 1000 A irrespective of processing conditions, but to a change in crystallite orientation. X‐ray diffraction measurements showed an increased {110} texture in devices processed without HCl. In addition, in the latter devices, dopant diffusion under the gate from the source and drain contacts was reduced by 7.6% to 12%. A linear relationship was observed between the average grain‐boundary trap density and the average lateral dopant diffusion length. These findings are explained by considering the point defec...

The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates

Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained‐layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep‐level transient spectroscopy showed that an n‐type majority‐carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α‐type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α‐type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not w...

Anisotropic strain relaxation of GaInP epitaxial layers in compression and tension

We have investigated the strain relaxation of intentionally lattice mismatched (±0.5%) GaInP layers grown on GaAs substrates by organometallic vapor phase epitaxy. Double axis x‐ray diffraction was used to measure the relaxation in these epitaxial layers in perpendicular 〈110〉 directions as a function of thickness. For samples in tension, the difference in relaxation between [110] and [110] increases from 10% to 48% as the layer thickness increases from 7 to 28 times the critical thickness, hc. For samples in compression this difference is 28% at 24hc while no relaxation is measured for a sample at 6hc. These results indicate that strain relaxes anisotropically and that the anisotropy is more pronounced for samples in tension than in compression. Furthermore, the major relaxation axis was found to be [110] regardless of the sign of the strain. Reciprocal space maps, generated using triple axis x‐ray diffraction, showed that the amount of microtilt of the epitaxial layers was also anisotropic. This aniso...

Kinetics of silicide-induced crystallization of polycrystalline thin-film transistors fabricated from amorphous chemical-vapor deposition silicon

Self-aligned, n-channel, polycrystalline silicon thin-film transistors with 15 μm channels were fabricated by recrystallizing amorphous silicon for 6 h at 500 °C. A thin nickel silicide at the source and drain was used to seed the crystallization. The channel mobility was initially 87 cm2/V s, and improved to 170 cm2/V s after hydrogenating the devices. The recrystallization velocity in the channel was measured optically and electrically, and found to be 3.5×10−8 cm/s; this value exceeds by two orders of magnitude the solid-phase epitaxial regrowth rate of amorphous silicon. This observation, together with the low activation energy of 0.3 eV measured for the silicide-assisted regrowth velocity as compared to 2.76 eV for epitaxial regrowth, suggest that the channel recrystallization is assisted by Ni diffusing to the recrystallization front.

Influence of lattice misfit on heterojunction bipolar transistors with lattice‐mismatched InGaAs bases

AlGaAs/InGaAs/GaAs heterojunction bipolar transistors (HBTs) with lattice‐mismatched InGaAs bases have been studied for determining the effect of misfit defects on device performance. Defect structures were investigated using scanning cathodoluminescence and transmission electron microscopy. Misfit dislocations were found to form at the bottom interface between n‐InGaAs and n‐GaAs in the collector. Electrical properties of lattice‐misfitted n‐GaAs/n‐InGaAs heterojunctions were studied using p+‐GaAs/n‐GaAs/n‐InGaAs diode structures. Deep‐level transient spectroscopy measurements revealed electron traps (Et=0.49–0.55 eV) and hole traps associated with the lattice‐misfitted interface. Devices showed high dc current gains in spite of the presence of misfit dislocations. Poorer saturation of collector current was observed with increasing defect density. HBTS with lattice‐mismatched InGaAs bases are considered to be relatively insensitive to the introduction of misfit dislocations in terms of dc characteristics.